This invention relates to a vapor generating system and more particularly to a sub-critical or supercritical once-through vapor generating system for converting water to vapor.
In general, a once-through vapor generator operates to circulate a pressurized fluid, usually water, through a vapor generating section and a superheating section to convert the water to vapor. In these arrangements, the water entering the unit makes a single pass through the circuitry and discharges through the superheating section outlet of the unit as super-heated vapor for use in driving a turbine, or the like.
Although these arrangements provide several improvements over conventional drum-type boilers, some problems have arisen in connection with starting up the generators, usually stemming from fluid at an undesirable quantity or condition being passed to the components of the system, resulting in excessive thermal losses, as well as mismatching of temperature of the throttle steam to the turbine inlet causing a decrease in turbine component life.
Earlier attempts to solve some of these problems included arrangements providing bypass circuitry for a portion of the fluid at a point in the flow circuitry between the vapor generating and superheating sections and/or between the superheating section and the turbine during start-up to apportion flow within the system and yet avoid the possibility of fluid at an undesirable quantity or condition being passed to the turbine. However, these arrangements resulted in very poor heat recovery, and, therefore, operate at a reduced thermal efficiency and, moreover, resulted in relatively unsuitable turbine throttle vapor conditions for rolling and bringing the turbine up to speed prior to loading.
Attempts to alleviate the latter problems included installing a division valve in the main flow path to divert flow to a bypass circuit including a flash tank separator located between the vapor generating section and the superheating section, or between a primary and finishing superheater in the superheating section. In these arrangements, the flash vapor from the separator is furnished to the superheating section or to the finishing superheater, and the drains from the separator are passed to a deaerator and/or high pressure heater. However, in these systems, the separator could often accommodate only a limited pressure, which was considerably less than the full operating pressure of the main pressure parts. Therefore, after start-up when turbine demands approaches pressures exceeding the design pressure of the separator, the separator had to be switched out of operation and flow to the turbine supplied directly from the main flow line upstream of the flash tank. However, this switch of flow often caused control difficulties and, in addition, caused a drop in enthalpy at the turbine since the flow source switched from a saturated vapor from the separator to a lower enthalpy water-vapor mixture from the main flow line. Therefore, in order to avoid pressure excursions and an uncontrolled significant temperature drop at the turbine throttle, the valve controlling flow to the turbine directly from the main flow line had to be opened very slowly, the firing rate had to be increased, and the separator outlet valve closed to slowly transfer the sources of turbine steam from the separator to the main flow line. This of course, resulted in a considerable expenditure of time and energy, and a considerable sophistication of controls.
Also, in these latter arrangements, when vapor formed in the separator in response to a start-up firing rate input, the vapor, in addition to flowing to the turbine, was routed to other areas of the system such as high pressure heaters and/or the condenser until a percentage of the final turbine load was achieved. Therefore, these arrangements required the use and operation of several valves which added to the labor and costs in the operation of the system.
In order to overcome the foregoing problem, it has been suggested to provide a separator or separators directly in the main flow line between the vapor generating section and the superheating section. However, some of these arrangements have proven to be costly due to the fact that a relatively large, thick walled separator, and associated components, have to be used. Also, in some of these arrangements the vapor initially formed in the separator is passed in a circuit bypassing the finishing superheater and the turbine during start-up, after which the flow is switched to the superheater and turbine, which also requires a control system utilizing a number of valves. In order to alleviate the latter problems, the system disclosed in U.S. Pat. No. 4,099,384, issued July 11, 1978, and assigned to the assignee of the present invention, includes a plurality of separators disposed in the main flow line between the vapor generating section and the superheating section and adapted to receive fluid flow from the vapor generating section during start-up and full load operation of the system. In this arrangement, the boundary walls of the furnace section of the generator are formed by a plurality of vertically extending tubes having fins extending outwardly from diametrically opposed portions thereof with the fins of adjacent tubes being connected together to form a gas-tight structure. During start-up the furnace operates at constant pressure and super-critical water is passed through the furnace boundary walls in multiple passes to gradually increase its temperature. The system requires the use of headers between the multiple passes to mix out heat unbalances caused by portions of the vertically extending tubes being closer to the burners than others or receiving uneven absorption because of local slag coverage, burners out of service, and other causes. The use of these intermediate headers, in addition to being expensive, makes it undesirable to operate the furnace at variable pressure because of probability of separation of the vapor and liquid phases within the header and uneven distribution to the downstream circuit. Still further, this type of arrangement requires a pressure reducing station interposed between the furnace outlet and the separators to reduce the pressure to predetermined values, and, in addition requires a relatively large number of downcomers to connect the various passes formed by the furnace boundary wall circuitry.
U.S. Pat. No. 4,116,168, issued Sept. 26, 1978 and assigned to the assignee of the present invention, discloses a vapor generating system designed to overcome the latter problems. In this system a start-up arrangement is provided which does not require the use of bypass circuitry incorporating a low pressure flash tank separator.
To achieve this a plurality of separators are utilized which together operate at full system pressure and thus eliminate the need for a relatively large, thick-walled separator, while enabling the turbine to be smoothly loaded at pressures and temperatures that constantly and gradually increase. The separators are connected in the fluid flow circuitry in a series flow relation with said vapor generating section and the superheating section for receiving fluid from the vapor generating section during start-up and full load operation of the system and separating the fluid into a liquid and a vapor for the start-up and low load operation. The separated vapor is passed in the fluid flow circuitry to the superheating section, and drain liquid flow circuit means are connected to the separating means for passing the liquid from the separating means. The vapor generating section includes a furnace section the walls of which are formed by a plurality of tubes having fins extending outwardly from diametrically opposed portions thereof, with the fins of adjacent tubes being connected together to form a gas-tight structure. A portion of the latter tubes extend at an acute angle with respect to a horizontal plane.
The disposition of the separator in the main flow circuit results in the elimination of bypass circuitry and valving. Also the use of the angularly extending tubes which wrap around to form the intermediate furnace section enables the fluid to average out furnace heat unbalances and be passed through the boundary walls of the furnace section in one complete pass, thus eliminating the use of multiple passes and their associated mix headers and downcomers. Also, as a result of the angularly extending tubes, a relatively high mass flow rate and large tube size can be utilized over that possible with vertical tube arrangements.
This arrangement, although resulting in the foregoing advantages, is not without problems. For example, since the boiler operates at variable pressure, which extend up to and through 2400-3000 psi range, it is important that a phenomenon known as nucleate boiling is maintained. Nucleate boiling is characterized by the formation and release of steam bubbles on the inside of the heat absorbing surface of the tubes with the water still wetting the surface. The latter is important since the wall metal temperature of a tube will not rise substantially above the temperature of the contained fluid enough to weaken or otherwise damage the tube so long as the tube is wet with water on the inner wall surface opposite the heat receiving outer surface, i.e., as long as nucleate boiling is taking place, even with high heat transfer rates through the metal of the tube wall due to the contact of hot gases and/or radiation from a furnace. However, in the relatively high pressure range set forth above, the desirable nucleate boiling in high heat flux zones tends to be replaced by what is commonly known as "film boiling", in which a steam film forms over the heat transfer surface and prevents the liquid from wetting the surface. The steam film thus acts as a layer of insulation which retards the heat being transferred from the heat absorbing surface to the water and therefore the metal temperature of the tube increases rapidly. The resulting metal temperature may be high enough to cause an immediate tube failure and if not, it promotes corrosion which can eventually cause the tube to fail. Therefore, it is especially important in these types of arrangements to insure that nucleate boiling is maintained during all operating conditions of the boiler. In the boiler disclosed in the latter cited application departure from nucleate boiling may be avoided if a relatively high fluid mass flow rate is maintained through the boiler.
Also in the boiler discussed above, the relatively high mass flow rate must be maintained at low loads since the angularly extending tubes promote separation of the steam and water phases during flow through the tubes--a decided disadvantage in these types of arrangements.
However, the maintenance of the high mass flow rates for the above reasons is not without problems since it results in a higher pressure drop across the circuit and requires the use of relatively small diameter tubes.